Mark sysroots vendor_available

As part of adding vendor support for Rust static libraries, we need to
make the sysroots (the rlib variants at least) vendor available.

Bug: 172525289
Test: New Soong tests pass
Test: Example vendor cc_binary links against rust_ffi_static module.
Change-Id: Ib2255c9ae11c670bd239fa0051f3fb6f85717e85
diff --git a/Android.bp b/Android.bp
index cff0984..59c5a79 100644
--- a/Android.bp
+++ b/Android.bp
@@ -12,6 +12,7 @@
     native_coverage: false,
     host_supported: true,
     sysroot: true,
+    vendor_available: true,
     apex_available: [
         "//apex_available:platform",
         "//apex_available:anyapex",